1. Field of the Invention
This invention is related in general to methods and devices for testing the condition of automotive brake fluid. In particular, the invention pertains to a novel and different approach based on a correlation between brake-fluid condition and the concentration of specific metals in the fluid, in particular copper, iron, and zinc.
2. Description of the Related Art
Brake fluid is used to transmit the pressure exerted on a motor vehicle""s brake pedal to the slave cylinders of the braking system. The most commonly used brake fluids consist of glycol-based liquids categorized as DOT3 and DOT4 on the basis of the boiling point resulting from their particular composition. In order to prevent boiling of the fluid caused by overheating during use, DOT3 and DOT4 fluids are required to have a dry boiling point (with no moisture in fluid) of at least 401xc2x0 F. (205xc2x0 C.) and 446xc2x0 F. (230xc2x0 C.), respectively, so that proper brake operation is ensured under all temperature conditions. A low boiling point can cause the brake fluid to vaporize under hot operating conditions, such as during continuous braking on a downhill road. Such vaporization can create pockets of compressible vapor in the system that dangerously reduce the effectiveness of the braking system. This is the occurrence normally referred to as xe2x80x9cbrake fadexe2x80x9d which, in extreme cases, can cause complete brake failure.
All types of glycol-based brake fluid are hygroscopic. As a result of this property, they readily absorb moisture that reduces their boiling point and, if unchecked, can become dangerous. Thus, replacement of DOT3 and DOT4 fluids is desirable when they have absorbed enough moisture to decrease their vaporization temperatures to about 284xc2x0 F. (140xc2x0 C.) and 311xc2x0 F. (155xc2x0 C.), respectively, which are minimum acceptable wet boiling points (with moisture absorbed by the fluid) empirically considered safe for brake operation and established as such by the U.S. Department of Transportation. Since the moisture content of brake fluid increases with age and exposure to ambient humidity, it is clear that it should be checked periodically and that the fluid should be replaced when its boiling point approaches these safety limits.
Copending U.S. Ser. No. 10/030,455, hereby incorporated by reference, discloses a novel approach for estimating the boiling point of brake fluid based on the discovery that moisture content is closely correlated to the copper content in the fluid. The invention consists of a reactive test strip adapted to measure and indicate the concentration of copper ions in brake fluid in terms of a readily visible color change. The strip is immersed in the fluid and the resulting color acquired by reacting with the fluid is compared to a copper concentration-versus-color chart or to a standard color representative of the maximum concentration determined empirically to correspond to a boiling point considered safe for normal operation. If the color change indicates a higher-than-desirable copper concentration, the brake fluid is considered inadequate for safe operation and replaced without further tests. A similar strip is disclosed for measuring iron concentration, which, as well as zinc, is another metal found to provide a useful correlation with the moisture content of the brake fluid.
Another important aspect of brake systems maintenance is the corrosive nature of some of their constituents, which, upon contamination of the brake fluid, progressively contribute to damage metallic tubing and other parts of the brake system. In conventional fluids, amines are added to inhibit corrosion and prevent damage to metal parts that operate in contact with the fluid. As the brake fluid ages, its anticorrosive properties are measured in terms of reserve alkalinity; that is, the amount of amines remaining in the fluid to buffer the acidity resulting from breakdown of fluid constituents. Over time, thermal oxidation and volatization produce a significant reduction of the amine content and the concurrent decrease of anticorrosive properties. Tests have shown that the reserve alkalinity of DOT3 and DOT4 fluids is reduced to about 20 percent of its original value after 18 to 20 months of normal operation. Therefore, brake fluids also need to be checked and periodically replaced in order to prevent dangerous corrosion in the brake system. Accordingly, industry maintenance recommendations are typically based on service time and milage of the vehicle.
This invention constitutes a further advance in the art based on the recognition that the copper content in brake fluid is predictably related not only to moisture content but also to time and mileage of vehicle operation. Thus, this correlation can be advantageously used to estimate milestones for maintenance purposes without regard to actual service time and/or mileage. Instead, copper content is adopted as a reliable indicator of a vehicle""s xe2x80x9cvirtual age,xe2x80x9d a term used to refer to the wear and tear on brake fluid resulting from actual mileage and/or time of service.
The primary objective of this invention is a method and a test kit for determining whether the brake fluid in a vehicle has reached a state of deterioration corresponding to a predetermined time and/or mileage considered to be a threshold warranting replacement as a matter of scheduled maintenance.
Another objective of the invention is a method and apparatus for assessing the condition of the fluid""s degree of contamination, and correspondingly its remaining anti-corrosive properties, with a test similar to the one performed to determine its boiling point.
Another goal is a test that does not require retrieval of a sample of fluid from the braking system and does not involve elaborate testing procedures.
Yet another goal is a procedure that can be carried out rapidly during regular automotive maintenance.
A final objective is a procedure that can be implemented easily and economically according to the above stated criteria.
Therefore, according to these and other objectives, a normalized correlation between the concentration of copper in brake fluid and its service age and/or mileage has been developed and utilized to estimate the degree of degradation of the fluid. In its preferred embodiment, the invention consists of a reactive test strip adapted to measure and indicate the concentration of copper ions in brake fluid in terms of a readily visible color change. The strip is immersed in the fluid and the resulting color acquired by reaction with the fluid is compared to a copper concentration-versus-color chart or to a color standard representative of the concentration determined empirically to correspond to the desirable xe2x80x9cvirtual-agexe2x80x9d threshold for fluid replacement or other maintenance. If the color change indicates a higher-than-acceptable copper concentration, the brake fluid is considered degraded and contaminated, and therefore also corrosive and in need of replacement for proper maintenance of the vehicle""s braking system. An automated embodiment of the invention includes an optical instrument to compare the color obtained from the test to a predetermined standard and determine whether a fluid change is recommended. According to other, less preferred, embodiments of the invention, similar correlations are derived between concentrations of iron and zinc and the virtual age of brake fluid.
Various other purposes and advantages of the invention will become clear from its description in the specification that follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the objectives described above, this invention consists of the features hereinafter illustrated in the drawings, fully described in the detailed description of the preferred embodiment and particularly pointed out in the claims. However, such drawings and description disclose only some of the various ways in which the invention may be practiced.